Dielectric, calorimetric and elastic anomalies associated with the first order phase transition in (Ca, Sr)TiO3 perovskites

Abstract

Conduction calorimetry has been used to determine with high precision the latent heat andvariation in heat capacity which accompany the first order phase transition in perovskites with compositions(Ca1−xSrx)TiO3,x = 0.65, 0.68, 0.74 (CST65, CST68, CST74). In CST65 (CST68), thelatent heat is dissipated/absorbed over a temperature interval of∼11 K (∼6 K), which iscentred on ∼292 K(∼258 K) duringcooling and ∼302 K (∼270 K) during heating. The magnitude of the latent heat diminishes with increasingSrTiO3 content and was not detected in CST74. Integration of the latent heat and excess heatcapacity yields small excess entropies, which are consistent with the structural changesbeing displacive rather than order–disorder in origin. Resonant ultrasound spectroscopymeasurements on the same CST65 sample as used for dielectric and calorimetric measurementsthrough the same temperature intervals have allowed quantitative correlations to bemade with the bulk modulus, shear modulus and acoustic dissipation parameter,Q−1. The dielectric anomalyand changes in Q−1 can be understood as being linear combinations of the properties of the separateI4/mcm andPbcm phases in proportion to their volume fractions across the two-phase field. A change of only∼0.5–1 GPa has been detected in the bulk modulus but the shear modulus softens by∼5–8 GPa as the transition interval is approached from above and below. This shear modesoftening presumably reflects clustering and/or phonon softening in both theI4/mcm andPbcm structures. This pattern of structure–property relations could be typical of first ordertransitions in perovskites where there is no group/subgroup relationship between the highand low symmetry phases.